Batteries are a useful source of stored energy that can be incorporated into a number of systems. Rechargeable lithium-ion batteries are attractive energy storage systems for portable electronics and electric and hybrid-electric vehicles because of their high specific energy compared to other electrochemical energy storage devices. In particular, batteries with a form of lithium metal incorporated into the negative electrode afford exceptionally high specific energy (in Wh/kg) and energy density (in Wh/L) compared to batteries with conventional carbonaceous negative electrodes.
When high-specific-capacity negative electrodes such as lithium are used in a battery, the maximum benefit of the capacity increase over conventional systems is realized when a high-capacity positive electrode active material is also used. Conventional lithium-intercalating oxides (e.g., LiCoO2, LiNi0.8Co0.15Al0.05O2, Li1.1Ni0.3Co0.3Mn0.3O2) are typically limited to a theoretical capacity of ˜280 mAh/g (based on the mass of the lithiated oxide) and a practical capacity of 180 to 250 mAh/g. In comparison, the specific capacity of lithium metal is about 3863 mAh/g. The highest theoretical capacity achievable for a lithium-ion positive electrode is 1168 mAh/g (based on the mass of the lithiated material), which is shared by Li2S and Li2O2. Other high-capacity materials including BiF3 (303 mAh/g, lithiated) and FeF3 (712 mAh/g, lithiated) are identified in Amatucci, G. G. and N. Pereira, Fluoride based electrode materials for advanced energy storage devices. Journal of Fluorine Chemistry, 2007. 128(4): p. 243-262. All of the foregoing materials, however, react with lithium at a lower voltage compared to conventional oxide positive electrodes, hence limiting the theoretical specific energy. The theoretical specific energies of the foregoing materials, however, are very high (>800 Wh/kg, compared to a maximum of ˜500 Wh/kg for a cell with lithium negative and conventional oxide positive electrodes).
Lithium/sulfur (Li/S) batteries are particularly attractive because of the balance between high specific energy (i.e., >350 Wh/kg has been demonstrated), rate capability, and cycle life (>50 cycles). Only lithium/air batteries have a higher theoretical specific energy. Lithium/air batteries, however, have very limited rechargeability and are still considered primary batteries.
One significant consideration in the incorporation of lithium-ion battery cells into a particular application is the manner in which lithium-ion battery cells react to overcharging and over-discharging conditions. Over-discharging a lithium ion battery cell can irreversibly damage the cell and significantly shorten its cycle life. Likewise, charging a lithium ion battery cell above a particular cutoff voltage, which is dependent upon the particular chemistry, can shorten the cycle life of the cell.
Specifically, overcharge or over-discharge of lithium-ion battery cells may result in the generation of H2, N2, and other gases depending upon cell chemistry. When a pouch or other flexible form of cell packaging is used for the battery, this gas generation may lead to a swelling of the battery, which can result in disconnection of cell layers and, in extreme cases, venting of the battery. Venting of gases may present a serious safety concern.
Various approaches have been developed to guard against overcharge or over-discharge of lithium-ion battery cells. By way of example, U.S. Pat. No. 6,046,575, issued on Apr. 4, 2000, is directed to a circuit designed to prevent overcharge or over-discharge of lithium-ion battery cells. Battery control circuits, however, can become disabled, particularly when the associated battery is located on a vehicle that has been involved in an accident.
What is needed therefore is a battery that exhibits a reduced reliance on external circuits to protect against venting of a lithium-ion battery cell as a result of overcharge or over-discharge.